Both type 1 and 2 diabetes are caused by inadequate insulin secretion, and in both diseases this can be ascribed in large part to loss of pancreatic ?-cells that produce insulin. Both diseases can be cured by replacement of pancreatic ?-cells by pancreas transplantation but this requires life-long immunosuppression and there are far fewer organ donors than people with diabetes. An alternative approach to restoring pancreatic ?-cells is to foster regeneration of ?-cells in the patient. Recent studies in mice have provided important insights into how ?-cells are formed and their number increased and maintained after birth. Complementary studies in humans are few because of the difficulty in obtaining appropriate material and the difficulty of undertaking lineage tracing studies in humans.